Electromagnetic (EM) tracking technology is increasingly being used in the field of image guided interventions and therapy (IGIT).
For instance, EP 1 504 713 A1 discloses an image guided navigation system for navigating a region of a patient, which includes an imaging device, a tracking device, a controller, and a display. The imaging device generates images of the region of a patient. The tracking device makes use of an EM tracking system to track the location of the instrument in a region of the patient and the controller superimposes an icon representative of the instrument onto the images generated from the imaging device based upon the location of the instrument.
EM spatial measurement systems determine the location of medical objects based on electromagnetic induction. Such medical objects are embedded with sensor coils or other EM sensors. When the medical object is placed inside controlled varying magnetic fields, voltages (potential differences) are induced in the EM sensors, e.g., coils. These induced voltages are used by the measurement system to calculate the position and orientation of the medical object.
Unfortunately, electromagnetic induction also occurs when the object is being moved through a static magnetic field. The induced potential difference in this scenario is proportional to the derivative of the magnetic flux, which in turn relates to the speed of relative movement between the medical object and the static magnetic field.
EM spatial measurement systems may determine the location and orientation of objects purely based on observed voltages and thus cannot distinguish between the sources of the potential difference. Thus, the positioning of a medical object in motion is—by definition—less accurate, compared to a static medical object. In most practical applications, this does not cause problems as the induced potential differences due to motion are relatively small compared to the induced potential difference generated by the magnetic field variation. As a result, an introduced measurement error is small compared to the noise of the measurement system.
However, when the movement of the medical object is fast, the additionally induced potential difference does introduce significant errors in positioning. If the motion is very fast, the potential difference may even exceed expected induction levels due to the magnetic field variation. In the latter scenario, EM systems report lost signals from the EM sensor. As a result, EM systems cannot be used for tracking fast moving objects.